1. Field of the Invention
The present invention relates to ballasts for fluorescent lamps, particularly of a kind wherein each lamp is powered by two separate current sources of different frequencies.
2. Elements of Prior Art
A power-line-operated electronic fluorescent lamp ballast typically consists of a power-factor-correcting input circuit and a frequency-converting power supply operable to supply the fluorescent lamp with a current of frequency substantially higher than that of the power line voltage, thereby attaining higher-than-normal lamp luminous efficacy.
One of the reasons for attaining this higher-than-normal lamp luminous efficacy is that of maintaining the ionization of the gas inside the fluorescent lamp at a substantially constant level; which indeed results when suppling the lamp with a high-frequency sinusoidally alternating current.
In contrast, when supplying the lamp with 60 Hz sinusoidally alternating current, the lamp's ionization level will vary approximately in proportion to the instantaneous magnitude of the lamp current; an effect which does not occur when the lamp is being powered with a current of frequency so high that the response-time of ionization is low in comparison with the duration of a half cycle of the high-frequency current.
However, the power-factor-correcting input circuit is apt to dissipate as much power as is saved by maintaining the lamp's ionization level constant; which therefore reduces the improvement in luminous efficacy otherwise attainable.